It is known that a personal computer employs hard disk drive (HDD) as a loading unit of the operation system. The hard disk drive is a non-volatile storage device based on rigid rotational disk. Digital data are stored on and searched from the smooth magnetic surface of the hard disk drive. The polarity is changed by the electromagnetic flow of a magnetic head, which is very close to the magnetic surface, so as to write the data onto the disk. The data can be read in a reverse manner. For example, the current of a coil is variable with a magnetic field when the magnetic head overpasses the coil.
Following the development of various electronic products, different standard sizes of hard disk drives have been developed and applied to the electronic products. For example, 0.85-inch hard disk drive is generally applied to a portable device such as a cellular phone, one-inch hard disk drive (MicroDrive) is generally applied to a digital camera (CF type II interface), 1.8-inch hard disk drive is generally applied to a notebook or an external hard disk cartridge, 2.5-inch hard disk drive is also generally applied to a notebook or an external hard disk cartridge, and 3.5-inch hard disk drive is generally applied to a desktop computer. The external hard disk cartridge for 3.5-inch hard disk drive needs to connect with an external power supply. This is inconvenient.
Lately, the hard disk drive is replaced with a permanent memory such as flash memory or a non-permanent such as synchronous dynamic random access memory (SDRAM). The external storage device based on such memory is referred to as solid state disk or solid state drive (SSD). The solid state disk is made with a configuration identical to that of a regular hard disk, for example, 1.8-inch, 2.5-inch or 3.5-inch hard disk drive. The solid state disk employs an interface compatible with that of the regular hard disk to replace the standard size of hard disk drive in the computer. There is no rotational disk-like structure in the solid state disk. However, such memory is still inherently named “hard disk”.
The solid state disk is advantageous in that it has low power consumption and low heat and is noise-free and anti-vibration. Accordingly, not only the data can be more securely stored, but also the continuous operation time of the battery-powered device can be prolonged. However, the popularization of the solid state disk is most limited by the cost and lifetime. No matter whether the solid state disk is the permanent memory or non-permanent memory, the cost of the solid state disk per million bytes is much higher than that of the hard disk drive. Moreover, the flash RAM has a fixed write lifetime. When the lifetime expires, the data cannot be read. This is another obstacle to the popular acceptance of the solid state disk. Furthermore, it is hard to repair a damaged solid state disk. In case the flash memory chip for storing data is damaged, it is impossible to recover the data from the damaged chip with the existent data reparation technique. In contrast, the data may be more or less recovered from a conventional mechanical hard disk drive.
The advantages and shortcomings of the hard disk drive as an operation unit and the flash memory as an operation unit are compared as follows:    1. The hard disk drive has a major advantage of large memory capacity. With respect to the specification of a current hard disk drive product, 500 GB and 640 GB memories have already been mass-produced and the memory capacity will be even larger in the future. Such memory capacity is sufficient to most of the users. However, in comparison with the flash memory as a storage unit, the hard disk drive has many shortcomings including great power consumption, high temperature and great noise. The most serious defect of the hard disk drive is its poor stability. The spindle motor of a 2.5-inch hard disk drive generally can operate at 5400 rpm or 7200 rpm or even 10000 rpm. When operating at high speed, the hard disk drive is very likely to damage due to collision or shock. Once the tracks are damaged, the data recorded on the tracks are very likely to totally or    2. The flash memory is mainly advantageous in that it operates at higher speed and is noise-free (since it needs no motor). The flash memory is disadvantageous in that it has too small capacity and is manufactured at too high cost. Currently, one single flash memory chip has a capacity of about 8 GB or 16 GB. Therefore, it needs 40 flash memory chips to provide a capacity of 640 GB. However, 16 GB or 32 GB is large enough to simply store the operation system and the ordinary application software.